PX4 User Guide (v1.14)

# CRSF Telemetry (TBS Crossfire Telemetry)

CRSF is a telemetry protocol that can be used for both RC control and to get telemetry information from the vehicle/flight controller on a compatible RC transmitter.

The protocol was developed by Team BlackSheep (opens new window) for their Crossfire RC system, but is also used by ExpressLRS (ELRS) (opens new window) RC systems. It is a bidirectional protocol that only needs a single UART for communicating both RC and telemetry.

The supported telemetry messages are listed here, and include: flight mode, battery level, GPS data RC signal strength, speed, altitude, and so on.

Note

If you don't need telemetry you can connect a TBS Crossfire to the RCIN port and configure the receiver to use S.BUS. Crossfire radio systems can also be used as Telemetry Radios.

WARNING

PX4 does not include the CRSF protocol support by default. The instructions below explain how to build and upload custom PX4 firmware that includes the required modules.

# Radio System Setup

To use CRSF telemetry you will need a TBS Crossfire radio system or ExpressLRS radio system that includes an RC controller with a transmitter, and a receiver (from the same vendor).

Note

An RC radio system historically consisted of a ground-based controller that transmitted to an on-vehicle receiver. Even though many radio systems are now bidirectional, the ground module may still be referred to as the transmitter, and the air unit may be called a receiver.

Generally you will need to separately setup and configure the transmitter and receiver, and then bind them together.

A transmitter might come as an integral part of an RC controller, or it might be a separate module that you plug into a controller. If it is a separate module then you may also need to update the module software on the transmitter to firmware that supports CRSF, such as OpenTX or EdgeTx. In both cases you will need to configure the transmitter to enable CRSF.

The receiver must be wired to a spare port (UART) on the Flight Controller. Then you can bind the transmitter and receiver together.

Instructions for the steps above are covered in

# Wiring

The TX and RX on your selected Flight Controller UART should be connected to separate channels on the receiver. The signal is usually uninverted, and can be directly connected (no additional inverter logic is required in the cable). You should check the manual for your specific receiver though!

# TBS Receiver Wiring

For TBS receivers you wire the FC UART and receiver as shown (this assumes the TBS Nano RX).

FC UART Nano RX
TX Ch2
RX Ch1

# ExpressLRS Receiver Wiring

For ExpressLRS receivers wire to the flight controller UART as shown below (wiring is covered in detail here (opens new window)):

FC UART ExpressLRS
TX RX
RX TX
VCC VCC
GND GND

# PX4 Configuration

# Firmware Configuration/Build

CRSF telemetry support is not included in any PX4 firmware by default. To use this feature you must build and upload custom firmware that includes crsf-rc and removes rc_input.

The steps are:

  1. Setup a development environment for building PX4.

    As part of this process you will have used git to fetch source code into the PX4-Autopilot directory.

  2. Open a terminal and cd into the PX4-Autopilot directory.

    cd PX4-Autopilot

  3. Launch the PX4 board config tool (menuconfig) for your make target using the boardconfig option (here the target is the ARK Electronics ARKV6X flight controller):

    make ark_fmu-v6x_default boardconfig

  4. In the PX4 board config tool:

    • Disable the default rc_input module
      1. Navigate to the drivers submenu, then scroll down to highlight rc_input.
      2. Use the enter key to remove the * from rc_input checkbox.
    • Enable the crsf_rc module
      1. Scroll to highlight the RC submenu, then press enter to open it.
      2. Scroll to highlight crsf_rc and press enter to enable it.

    Save and exit the PX4 board config tool.

  5. Build the PX4 source code with your changes (again assuming you are using ARKV6X):

    make ark_fmu-v6x_default

This will build your custom firmware, which must now be uploaded to your flight controller.

# Firmware Upload

To upload the custom firmware, first connect your flight controller to the development computer via USB.

You can upload firmware as part of the build process using the upload options:

make ark_fmu-v6x_default upload

Alternatively you can use QGroundControl to install the firmware, as described in Firmware > Installing PX4 master, beta, or custom firmware.

# Parameter Configuration

Find and set the following parameters:

  1. RC_CRSF_PRT_CFG — Set to the port that is connected to the CRSF receiver (such as TELEM1).

    This configures the serial port to use the CRSF protocol. Note that some serial ports may already have a default serial port mapping or default MAVLink serial port mapping that you will have to un-map before you can assign the port to CRSF. For example, if you want to use TELEM1 or TELEM2 you first need to modify MAV_0_CONFIG or MAV_1_CONFIG to stop setting those ports.

    There is no need to set the baud rate for the port, as this is configured by the driver.

  2. RC_CRSF_TEL_EN — Enable to activate Crossfire telemetry.

# Radio Setup

Radio Control Setup explains how to map your RC controller's attitude control sticks (roll, pitch, yaw, throttle) to channels, and to calibrate the minimum, maximum, trim and reverse settings for all other transmitter controls/RC channels.

# RC Controllers

A transmitter might come as an integral part of an RC controller, or it might be a separate module that you plug into a controller.

RC Controllers that support TBS Crossfire and ExpressLRS TX modules:

  • FrSky Taranis X9D Plus (opens new window) has an external module bay that can be used with TBS or ExpressLRS transmitter modules that are "JR module bay" compatible. You will need to install OpenTX software, which supports CRSF, and enable the external module and CRSF.
  • Radiomaster TX16S (opens new window) has an internal ExpressLRS transmitter module. It also has an external module bay that can be used with TBS or ExpressLRS transmitter modules that are "JR module bay" compatible. It runs both OpenTX and EdgeTx software, either of which can support CRSF.

# TBS Radio Systems

TBS Crossfire Radio Systems are listed here (opens new window). A few options are listed below.

Transmitter modules:

Receivers:

# ExpressLRS Radio Systems

Express LRS provide Radio System guidance in the Hardware Selection (opens new window) page. A few tested options are listed below.

Transmitter modules:

  • TBD

Receivers:

# Telemetry Messages

The supported telemetry messages and their source are listed below (this table is reproduced from the TBS Crossfire Manual: "Available sensors with OpenTX" (opens new window)).

Datapoint Description Data source
1RSS Uplink - received signal strength antenna 1 (RSSI) TBS CROSSFIRE RX
2RSS Uplink - received signal strength antenna 2 (RSSI) TBS CROSSFIRE RX
RQly Uplink - link quality (valid packets) TBS CROSSFIRE RX
RSNR Uplink - signal-to-noise ratio TBS CROSSFIRE RX
RFMD Uplink - update rate, 0 = 4Hz, 1 = 50Hz, 2 = 150Hz TBS CROSSFIRE RX
TPWR Uplink - transmitting power TBS CROSSFIRE TX
TRSS Downlink - signal strength antenna TBS CROSSFIRE TX
TQly Downlink - link quality (valid packets) TBS CROSSFIRE TX
TSNR Downlink - signal-to-noise ratio TBS CROSSFIRE TX
ANT Sensor for debugging only TBS CROSSFIRE TX
GPS GPS Coordinates TBS GPS / FC
Alt GPS Altitudes TBS GPS / FC
Sats GPS Satellites acquired TBS GPS / FC
Hdg Magnetic orientation TBS GPS / FC
RXBt Battery voltage TBS GPS / FC/ CROSSFIRE RX/ CORE
Curr Current draw TBS GPS / FC// CORE
Capa Current consumption TBS GPS / FC/ CORE
Ptch FC Pitch angle FC
Roll FC Roll angle FC
Yaw FC Yaw angle FC
FM Flight mode FC
VSPD Barometer FC

# See Also

← FrSky Telemetry →